Companion for Decouple Staking and Election - Part 3: Signed Phase (#2793)

* Companion for Decouple Staking and Election - Part 3: Signed Phase

https://github.com/paritytech/substrate/pull/7910

* remove some config types

* allow up to 5 signed submissions on polkadot and kusama

* signed phase is equal induration to unsigned phase

* use chain defaults for base and per-byte deposits; >= 16 SignedMaxSubmissions

* use a small but non-trivial solution reward

* reduce signed deposit per byte fee

* reduce signed reward, adjust polkadot expected soln size

* copy submit benchmark from substrate

* demo calculating an appropriate fee for the signed reward

Unfortunately, this doesn't work: it needs to be a constant function,
and AFAIK there's no way to make a trait method constant.

* SignedRewardBase is 1.5x the fee to submit a signed solution

* all chains use deposit byte of base per 50k

* update Substrate

* cargo update -p pallet-election-provider-multi-phase

Co-authored-by: parity-processbot <>